1. Field of the Invention
The present invention generally relates to a transmission type X-ray tube and a reflection type X-ray tube. More particularly, the present invention relates to a transmission type X-ray tube and a reflection type X-ray tube using filter materials to filter out unwanted radiation.
2. Description of Related Art
It is well know in the art of medical imaging that by using low Z filters such as aluminum, molybdenum, yttrium and copper the amount of low energy radiation can be reduced by what is referred to an aluminum equivalent filter thickness. Typically such thickness would range from 0.5 to 12 millimeters of equivalent aluminum filter which filters out x-rays of low energy, long wavelength, reducing potentially harmful and unnecessary radiation especially for medical imaging. Unfortunately such filters also filter out a large portion of the useful x-rays.
Non-destructive testing usually does not employ filters but when imaging of a specific Kα line emission from the target of the x-ray tube provides a high quality image of the object to be imaged in non-destructive testing, removal of unwanted high energy photons which cause loss of energy quality is also an objective of the current invention.
In medical imaging chemical imaging agents, such as iodine, gadolinium and barium based compounds, produce high contrast with respect to surrounding soft tissue because of their densities and atomic numbers. The significance of their atomic numbers (Z=53 for iodine, Z=56 for barium and Z=64 for gadolinium) is that the k-absorption edge is located at very favorable energies relative to the typical x-ray energy spectrum. The K edge for iodine is at 33.17 keV, is at 37.44 keV for barium and is at 50.24 kev for gadolinium). Maximum contrast is produced when the x-ray photon energy is slightly above the K-edge energy of the chemical imaging agents.
The selection of an optimum spectrum for a specific clinical procedure must take into consideration not only the requirements for contrast but also produce the necessary penetration through the body section and limit the radiation dose to the patient.
In the case of non-destructive imaging of various industrial products including but not limited to electronic circuit boards of all kinds, integrated circuits, LED's and lithium batteries, there is a single optimum energy for maximum image quality. However, in order to produce such a high flux optimum energy, inevitably higher energy photons above the optimum energy are produced at the same time. Such high energy photons are unwanted as they decrease image contrast. Sensor overload is a problem when too many x-rays not essential to making the image impinge on the sensor.
For a reflection type x-ray tube the spectrum of an x-ray beam is determined by combinations of the anode material, the filter material and thickness, and the selected electron tube voltage for the procedure. The thickness of the target is not a significant issue.
What is needed for x-ray imaging applications is an x-ray spectrum with a high number of photons in a narrow, well defined band of x-ray photon energies to provide high image contrast and a way to filter out those photons with energies higher and/or lower than the energy band while only minimally reducing the flux in said energy band is required to maximize image quality. The ratio of flux in the useful energy band to the energy above such band should be maximized within the limitations of thermal management of the x-ray tube. For medical imaging applications a way to simultaneously decrease the unnecessary low energy photons significantly reducing dose to the patient would provide a significant added benefit. For imaging of inanimate objects the photon energies can be as low as 15 to 20 kev while for general medical imaging would start closer to 30 kev and be as high as 600 kev for high energy imaging.
Such a filtering scheme is applicable to both reflection type and transmission type of x-ray tubes. When transmission tubes are used what is needed is way to optimize the ratio of useful x-rays to the amount of high energy photons above the useful x-ray band. In medical applications what is needed is a way to optimize the ratio of useful x-rays to the dose the patient receives while at the same time reducing the number of high energy photons above the useful band. Reflection tubes do not allow for optimization of flux using target thickness and hence are limited to adjusting the thickness and composition of the filter material to provide the same desired results.